1. Field of the Invention
The present invention relates in general to Satellite Positioning System (SATPS) receivers, and in particular to a network assisted pseudolite acquisition for enhanced SATPS navigation.
2. Description of the Related Art
Cellular telephony, including Personal Communication System (PCS) devices, has become commonplace. The use of such devices to provide voice, data, and other services, such as internet access, has provided many conveniences to cellular system users.
A current thrust in the cellular and PCS arena is the integration of Global Positioning System (GPS) technology, which is a subset of SATPS technology, into cellular telephone devices and other wireless transceivers. For example, U.S. Pat. No. 5,874,914, issued to Krasner, which is incorporated by reference herein, describes a method wherein the basestation (also known as the Mobile Telephone Switching Office (MTSO)) transmits GPS satellite information, including Doppler information, to a remote unit using a cellular data link, and computing pseudoranges to the in-view satellites without receiving or using satellite ephemeris information.
This current interest in integrating GPS with cellular telephony stems from a new Federal Communications Commission (FCC) requirement that cellular telephones be locatable within 20 feet once an emergency call, such as a “911” call (also referred to as Enhanced 911 or “E911”) is placed by a given cellular telephone. Such position data assists police, paramedics, and other law enforcement and public service personnel, as well as other agencies that may need or have legal rights to determine the cellular telephone's position. Further, GPS and/or SATPS data can be used by the cellular user for directions, location of other locations that the cellular user is trying to locate, determination of relative location of the cellular user to other landmarks, directions for the cellular user via internet maps or other GPS/SATPS mapping techniques, etc. Such data can be of use for other than E911 calls, and would be very useful for cellular and PCS subscribers.
However, since cellular telephones can travel into areas where SATPS signals cannot be reliably received, augmentations to the SATPS system are being researched to support the E911 and other SATPS/cellular applications. SATPS is increasingly being pressed into service in the cellular telephone/PDA/mobile computer application where a solution is required in areas with substantial blockage, such as inside buildings, in subway stations, and other areas where the system RF link budget is unable to sustain communications with mobile units that travel into hostile signal reception environments such a buildings. Pseudolites are well-known commercially available ground-based transmitters which augment the orbiting SATPS constellation with one or more additional transmitters to improve the availability and quality of a SATPS solution. Current pseudolite applications include local-area augmentation system (LAAS) transmitters for precision approach.
It can be seen, then, that there is a need in the art for a method and apparatus for assisting the cellular network in locating cellular telephones. It can also be seen that there is a need in the art for a method and apparatus for assisting the network in locating cellular telephones in hostile signal environments such as buildings. It can also be seen, then that there is a need in the art for methods and apparatuses for assisting a mobile user in a harsh signal environment using extra-network devices such as pseudolites.